The photopositive response of the frog (Rana pipiens) under photopic and scotopic conditions.

Under suitable experimental conditions (Boycott, Mrosovsky & Muntz, 1964) frogs jump towards the light (Pearse, 1910; Muntz, 19626). This response, which is released most readily by blue light, appears to be a function of the retinal projection to the diencephalon (Muntz, 1962 a, b). The responsiveness to blue is very marked, and is largely independent of the intensity and saturation of the stimulating lights. It was also possible to show that inhibition was involved, since adding green light to blue reduced its effectiveness (Muntz, 1962 J). As a result of further experiments (Muntz, 1963 a, b), it was suggested that the response was due to a synergistic action of the green rods and the cones, opposed by an inhibitory action from the red rods. This hypothesis is described in more detail in the Discussion. If the inhibitory effect is due to the red rods, dark-adaptation should have a very marked effect on the spectral response curve, since it will accentuate the contribution of the inhibitory receptor to the behaviour. This should result in a decreased responsiveness of the animals to light, and also in an accentuation of the effectiveness of blue light relative to other colours. Boycott et al. (1964) reported differences in the behaviour of dark-adapted and light-adapted frogs to light. The data in that experiment were, however, insufficient to test the predictions made above.